1. Field of the Invention
This invention relates to an image forming apparatus (electrophotographic apparatus) such as a copying machine, a printer, a facsimile machine or a platemaking system, which employs an electrophotographic process.
2. Related Background Art
In recent years, various approaches are taken because of an increasing need for the achievement of ultrahigh image quality in regard to images reproduced from the image forming apparatus. In particular, the exposure process that forms an electrostatic latent image on the surface of an electrophotographic photosensitive member is positioned on the upstream side in the electrophotographic process, and is the basis of image formation. Accordingly, making beam spot diameter small in the exposure process enables achievement of ultrahigh resolution, and is a very effective means for the achievement of ultrahigh image quality.
Near infrared region semiconductor lasers having conventionally been used have lasing wavelengths of about 650 to 780 nm, and have spot diameter of about 100 μm. Its limit has been about 50 to 80 μm whatever improvements are made on various optical members in order to make the beam spot diameter small. Also, even if improvements on various optical members have made the beam spot diameter small, it is difficult to obtain the sharpness of a contour of the beam spot. This is known from the diffraction limit of laser beams that is represented by the following equation (9). The following equation (9) shows that the lower limit of beam spot diameter (D) of a beam spot is proportional to the wavelength (λ) of the laser beam. (NA is the numerical aperture of a lens.)D=1.22λ/NA  (9)
Accordingly, it is contemplated to use as an exposure light source (a writing light source) of the electrophotographic apparatus a short-wavelength blue (purple) semiconductor laser (hereinafter simply “blue semiconductor laser”), which is being put into practical use in DVD and so forth in recent years (see, e.g., Japanese Patent Application Laid-open No. H9-240051, page 2, claim 1). Compared with the conventional near infrared region semiconductor lasers, in the case when the blue (purple) semiconductor laser having about a half lasing wavelength (380 to 450 nm) is used as an exposure light source, the beam spot can be made to have a fairly small spot diameter in the state the sharpness of the contour of the beam spot is maintained, as shown in the above equation (9). Hence, this enables achievement of ultrahigh resolution, and is very advantageous for the achievement of ultrahigh image quality.
Thus, the use of the blue (purple) semiconductor laser as an exposure light source makes it possible for the surface of an electrophotographic photosensitive member to be irradiated with a laser beam in a spot diameter of about 40 μm or less in the state the sharpness of its contour is maintained.
In an electrophotographic apparatus having such a blue (purple) semiconductor laser as an exposure light source and made to have a small beam spot diameter, an electrophotographic photosensitive member having a certain or higher sensitivity to light irradiation of an image exposure device is required as a matter of course. Further, in order for the electrophotographic photosensitive member to effectively utilize the light with which it is irradiated, the photosensitive member is required to have a maximum spectral sensitivity at wavelengths of about 380 to 520 nm. However, very few electrophotographic photosensitive members have such a maximum spectral sensitivity at the wavelengths of about 380 to 520 nm. For example, Japanese Patent Application Laid-open No. H10-239956, page 5, discloses a report concerning a selenium (Se-Te) photosensitive member which is an inorganic photosensitive member having a maximum spectral sensitivity at a wavelength of about 460 nm.
Meanwhile, in these days, various studies are made which take note of application of the blue semiconductor laser to organic photosensitive members which have various advantages that they have a small environmental load, can be manufactured and handled with ease and enjoy a low cost. For example, Japanese Patent Application Laid-open No. H10-239956, page 3 and FIG. 4 on page 6, discloses an image forming apparatus in which a photosensitive member having a maximum spectral sensitivity at wavelengths of 600 nm or less is used in combination with a laser diode which emits a short-wavelength laser beam. It shows an embodiment which makes use of an organic photosensitive member making use of a perylene type or azo type pigment as a charge-generating material and having a maximum spectral sensitivity at wavelengths of 540 to 580 nm. In this case, in respect of spectral sensitivity, a light source having a wavelength of about 640 nm or more (a red LED or the like) is considered usable which is conventionally used in destaticizers (charge elimination devices). However, organic photosensitive members can not be said to be effectively used in respect to the lasing wavelengths of 380 to 450 nm the blue (purple) semiconductor laser has. In an attempt to make the amount of laser light extremely large to secure sensitivity in order to more improve their sensitivity, the running potential may vary so greatly as to be insufficient for the reproduction of stable images with ultrahigh image quality throughout their running. At the same time, they also involve various disadvantages that the reliability of lasers to reproduction stability may lower, a high laser cost may result and the laser may have a short lifetime. Moreover, there is a limit to laser power, and proper sensitivity can not always be secured.
On account of the foregoing, it is sought to use an organic photosensitive member having a maximum spectral sensitivity at the wavelengths of 380 to 520 nm. However, as stated above, it is very difficult in regard to materials to design an organic photosensitive member which can effectively used in respect to the lasing wavelengths of 380 to 450 nm the blue (purple) semiconductor laser has. In addition, it has newly been found that, where such an organic photosensitive member having a maximum spectral sensitivity at the wavelengths of 380 to 520 nm is used and is used at a proper amount of light of the blue (purple) semiconductor laser, although it can be said to be effectively used in respect to the laser irradiation light, there is a technical problem that the running potential varies greatly. Hence, even the organic photosensitive member having a maximum spectral sensitivity in the wavelength region of the blue (purple) semiconductor laser has been insufficient for the reproduction of stable images with ultrahigh image quality throughout its running.